Spotlight with Halo applied is creating rings around light
That is a dithering problem. Which is not really a problem at all, but rather what physically must happen. That occurs any time there is a small change in color spread over a large area.  

One of the ways to get rid of the banding is to make a stronger contrast, such as a pure black background.

The best way to deal with it. Is under the Render tab in the Properties window. Set the dithering amount up a bit until the color banding is not visible.